1. Field Of The Invention
This invention is related to methods and apparatuses for measuring the strength and deformation of materials under triaxial extension or compression and in one aspect to a triaxial test device including a coupling interposed between a load piston and an end cap on a test sample.
2. Description of Related Art
Triaxial compression and extension tests provide important data for the modeling of rock behavior around underground openings, in underground fluid reservoirs, around surface excavations, and beneath surface structures. With a conventional triaxial test apparatus for rock testing, a test is performed inside an enclosed housing or hydraulic cell. A confining fluid is pumped into the test cell to provide a confining pressure on a rock sample specimen. The sample is placed on a bottom end cap for support. Hydraulic pressure is applied around the sample during the test to generate an isostatic confining stress. Then an axial load is applied (e.g. by a load piston) to a top end cap to generate a deviatoric (shear) stress. During such testing, fluid leakage between the top end cap and the load piston is possible. When a sample is directly jacketed to a load piston to prevent such leakage, correct mounting and positioning of the sample and accurate measurements can be difficult. Fragile samples, such as shales and weak sandstones that are usually expensive and difficult to obtain or prepare, are often damaged in a test set up and jacketing process. In certain prior art apparatuses, load piston diameter should match sample diameter and thus a different test cell is required for each different diameter sample to be tested. The lateral stress in triaxial compression or extension tests (see FIGS. 1A and 1B) is applied by a pressurized fluid which is prevented from entering the sample by an impermeable jacket. If the axial stress is to be less than the lateral stress, as required for triaxial extension, then this fluid is not allowed to apply a pressure on the sample anywhere in the axial direction. The axial stress is applied solely by a loading piston.
Strengths and deformations are usually measured by subjecting a right circular cylinder test sample to triaxial compression conditions. As illustrated in FIG. 1A, triaxial compression is defined as a state in which two of the three principal compressive stresses are less than the third. Triaxial extension, illustrated in FIG. 1B, is a state in which two of the three principal compressive stresses are greater than the third. All stresses are still compressive, however. In contrast to metals, there is evidence that the strength of geologic materials is different under triaxial extension than under triaxial compression. It is important to know the strength under both conditions because they are the two extreme compressive stress states that can occur. Around a wellbore, for example, the stress state can lie anywhere between triaxial compression and triaxial extension. If both strengths are known, then a more accurate extrapolation of laboratory measured strengths to downhole conditions is possible.
FIG. 2 illustrates one prior art triaxial test technique which allows triaxial extension to be performed. The sample is directly jacketed to the load piston. The sample may be glued to the piston. With these methods the test cell design is usually such that radial and axial strains are measured with strain gauges mounted on the sample which are prone to errors and localized non-representative measurements. Axial strain has sometimes been determined using a single measurement of the piston displacement outside the test cell. However, this makes it impossible to separate piston slack movement from actual sample deformation, and a single displacement measurement can be affected by tilting. For the technique illustrated in FIG. 2, axial stress is usually measured outside the test cell. This causes errors due to friction around the load piston. Also, the piston diameter should match the sample diameter, so a different test cell is required for each diameter to be tested. If this is avoided by using a tapered piston, then the confining pressure has a large effect on the true axial stress. It is often difficult to accurately compensate for this.
Methods that have been used to perform triaxial extension for soils testing are illustrated in FIGS. 3A, 3B, and 3C. Only one of these methods uses a load cell located somewhat close to the sample (FIG. 3B); however, it is not an integral part of the coupling and therefore may respond to the confining pressure as well as to the axial stress. Also, the techniques illustrated were developed for testing of soils and therefore may be limited to pressures much lower than those required for typical rock testing. The methods illustrated in FIGS. 3A and 3C allow confining fluid pressure to act downward on the top end cap, but this can be overcome by means of the rigid connection between the top end cap and the axial loading system.
In prior U.S. application Ser. Nos. 07/671,367 filed Mar. 19, 1991, now U.S. Pat. No. 5,265,461; and Ser. No. 07/913,853 filed Jul. 15, 1992 (both co-owned with this application), a triaxial test apparatus is disclosed which has a top load cell mounted on a top end cap. The load cell has a self-centering steel ball disposed partially within a recess in a cylindrical steel frame. The ball puts little or no torque on a sample to be tested and provides a relatively high stress at a center point of the frame, permitting sensitive accurate load measurement. A diaphragm strain gauge is attached in a slot in the bottom of the steel frame at a point beneath the point at which the ball contacts the frame. A load piston contacts the ball to impart a load through the top end cap to a sample. A plastic cap fits snugly over the frame and has a hole in its top through which the ball protrudes. The cap prevents the ball from coming out of the frame. A typical conventional diaphragm gauge (e.g. those provided by Micromeasurements Co.) may be used. Such gauges are accurate to about 2 to 10 psi in a load range of about 6000 to 8000 psi. The gauge's range can be increased to about 15,000 by changing the thickness of the cubical frame around the slot (e.g. from 0.1 inch to 0.15 inch) and/or by using a diaphragm gauge with a higher range capability. Wiring extends from the strain gauge, through a test cell housing to interface with a monitoring and/or recording and/or controlling system. Use of the ball deals with problems associated with matching load piston and sample diameters; but it does not allow for triaxial extension testing.